The basic structure of high electron mobility transistor (HEMT) consists of a modulation-doped heterojunction and source-drain structure. The two-dimensional electron gas (2-DEG) existing in modulation-doped heterojunction is free from the ionized impurity scattering effects and thus exhibits very high mobility. HEMT is a voltage controlled device. The gate voltage Vg controls the depth of the potential well of the heterojunction, and thereby controlling the surface density of 2-DEG in the potential well which in turn controls the operating current of the device. In the GaAs HEMT structure, usually the n-AlxGal-xAs control layer is depleted. If the n-AlxGal-xAs layer is thicker and doping level is high, then there are 2-DEG even when Vg=0, thus a depletion mode device. Otherwise, it is an enhancement mode device (when Vg=0 the Schottky depleted layer extends to the internal of the intrinsic GaAs layer); for HEMT, it is mainly controlling the wide band gap semiconductor layer (the control layer) doping and thickness, and controlling thickness in particular. When considering the 2-DEG surface charge density Ns of HEMT, one usually only need to consider the two 2-dimensional band (i=0 and 1) of the heterojunction potential well. The 2-DEG surface charge density Ns will be controlled by the gate voltage Vg.